We study broadening and the anomalously small shift of a 3.39-JU rotation-vibration line of methane at millitorr pressures 0 Saturation by the laser intracavity field allows investigation of the very sharp natural linewidth, without Doppler broadening. Two lasers were independently locked to this transition with an offset of less than ±1 kHz, a reproducibility of ±1 x 10 u .
A cw dye laser system frequency stabilized to a high-finesse optical reference cavity is described. Laser frequency is servo controlled to the cavity resonance with residual fluctuations less than 50 kHz for short times (20 psec) and 100 Hz for long times (10 sec). Drift in absolute laser frequency of about 1.5 MHz/min is observed due to drift of the unstabilized reference cavity. A saturated absorption spectrum of I2 obtained with this system is shown.
The photon-recoil components of the Ca 6573-A line have been resolved using the three-zone optical Ramsey interference technique with an atomic beam. Linewidths as narrow as 3 kHz HWHM (line Q of 8 X 10'0) are reported for zone separations up to 7 cm. An indication of the lightshift-induced contraction of the recoil splitting predicted by C. J. Bordii is obtained. Techniques are discussed which should lead to an optical wavelength/frequency standard with an accuracy of better than 10-l4.
The wavelength of the 3.39-pm line of methane has been measured with respect to the K P 6057-8, standard by using a frequency-controlled Fabry-Perot interferometer. W e have exhaustively studied systematic offsets inherent in the experiment, including effects due to asymmetry of the Kr standard line. Lacking a convention relating the defined Kr wavelength 6057.802 105 8, to observables of the krypton line (e.g., center of gravity or fringe maximum intensity point), we report two methane wavelengths: X, '= 33 922.31404 %, and A= =33 922.31376 A. Both results have an uncertainty of 6A= il. 2 x 10" 8, or 6A/A= i 3. 5 x lo4. Multiplication by the frequency measurement of the preceding letter gives the speed of light, c-= 299 792 456.2 (1.1) m/sec.
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